1. Field of the Invention
The present invention relates to handoff control methods, base station controllers, and base transceiver subsystems in mobile radio communication systems.
2. Description of the Related Art
In a CDMA radio communication system, radio waves sent from an antenna of a base transceiver subsystem to an access terminal diminish as the distance from the base transceiver subsystem increases. The radio waves finally do not reach the access terminal. The reach of radio waves is handled as a service area, and communication services are provided for a plurality of access terminals within the service area.
In the communication system described above, the access terminal must transfer its own location information to the network. Accordingly, the terminal always sends its current location information to the base transceiver subsystem. The base transceiver subsystem holds the location information sent from the plurality of access terminals within the local zone and enables data communication with the network. If the access terminal moves toward an adjacent zone during data communication, radio waves sent from the current base transceiver subsystem are gradually attenuated, radio interference from the adjacent base transceiver subsystem increases, and the quality of service decreases. Then, the communication is switched to the adjacent zone that can provide a good radio environment (the radio channel is switched), so that stable communication is maintained. This technology is referred to as a handoff control method.
A representative handoff control method is soft handoff. When the access terminal moves in the middle of communication, the soft handoff method establishes a radio channel to an adjacent base transceiver subsystem while maintaining the radio channel to the current base transceiver subsystem, and allows communication with a base transceiver subsystem that provides a better radio environment.
One soft handoff method disclosed in patent document 1 (Japanese Unexamined Patent Application Publication No. 2001-238248) is outlined as follows: One access terminal (AT) can simultaneously connect radio channels to a plurality of base transceiver subsystems (BTSs); The strength of a pilot signal from each BTS is always monitored; When the signal strength exceeds a given threshold, handoff to the corresponding BTS is carried out.
The conventional handoff control methods judge that a radio environment is good if the pilot signal is sufficiently strong.
The AT monitors a carrier-to-interference-power ratio (C/I), or a ratio of the strength C of the pilot signal to the interference signal I from the adjacent BTS. The AT specifies the transmission rate (throughput) of a forward link channel (from the base transceiver subsystem to the access terminal) in accordance with C/I. The AT sends a data rate control signal (DRC) including data rate information that requests the base transceiver subsystem to perform data communication at the specified throughput. The BTS that receives DRC specifies the highest possible transmission rate and starts communicating with the AT accordingly.
The radio signal used in the radio channel includes control data such as the pilot signal and traffic data such as voice conversation data and moving picture data. The traffic data is hereafter referred to as communication data.
FIG. 1 shows the configuration of a conventional CDMA radio communication system.
FIG. 1 shows that an AT which is receiving a Pilot CH1 signal from BTS1 101 in a position 105-1 moves in a first zone 103 toward BTS2 102, and reaches a position 105-2. The position is on the boundary of the first zone 103 and a second zone 104, and a Pilot CH2 signal from BTS2 102 can also be received there. A solid line representing radio waves indicates that the radio signal is received with a high strength while a broken line indicates that the strength is low.
If C/I of the Pilot CH2 signal from BTS2 102 exceeds a channel assignment threshold T_ADD when the AT is going to move from the position 105-2 to a position 105-3, the AT notifies a base station controller (BSC) 106 that the Pilot CH2 signal of BTS2 102 becomes a candidate for handoff, in the position 105-2 through BTS1.
The BSC 106 receives the notification and starts allocating a radio channel between the AT in the position 105-2 and BTS2 102. Now, the AT has two radio channels: one connected to BTS1 101 and the other connected to BTS2 102 (two-way state). Communication data is transferred in either of the two radio channels.
When the AT moves further to the position 105-3, C/I of BTS2 102 exceeds C/I from BTS1 101, and the communication is switched to BTS2 102. When the AT reaches a position 105-4, C/I of BTS1 101 falls below a communication capability threshold T_DROP. The Pilot CH1 signal of BTS1 101 is eliminated from the candidates for handoff, and the corresponding radio channel is disconnected. Then, the AT in the position 105-4 can communicate with BTS2 102 alone.
In the conventional handoff control system, the access terminal gives high priority to the strength of radio waves, and handoff is carried out to obtain a higher momentary transmission rate. No consideration is given to load statuses such as the total throughput and the number of channels connected to the base transceiver subsystem of the handoff destination. If the base transceiver subsystem of the handoff destination is congested, the user throughput may often decrease due to transmission scheduling on the side of the base transceiver subsystem, despite a good radio environment.
Moreover, because handoff control is performed irrespective of the congestion of the base transceiver subsystem, imbalances may be created among access terminals handed off to base transceiver subsystems.